1. Field of the Invention
The present invention concerns a switch having a housing which receives a temperature-dependent switching mechanism and has a lower part on whose inner bottom a first countercontact for the switching mechanism is arranged, and comprises a cover part, closing off the lower part, on whose inner side a second countercontact for the switching mechanism is provided, the switching mechanism comprising an electrically conductive spring disk which carries a movable contact element and operates against a bimetallic snap disk which sits approximately centeredly on the movable contact element, the spring disk being braced at its rim against one countercontact and pressing the movable contact element against the other countercontact when the switching mechanism is below its response temperature.
2. Related Prior Art
A switch of this kind is known from DE 37 10 672 A1.
In the case of the known switch, the housing has a lower part produced from electrically conductive material and a cover part, closing off the lower part, that is produced from insulating material. Arranged in said housing is the switching mechanism, which comprises a spring disk which carries a movable contact element. The spring disk operates against a bimetallic snap disk which is slipped over the movable contact element. Below the switching temperature, the spring disk, which is braced against the bottom of the lower part, presses the movable contact element against a countercontact which is provided internally on the cover part and extends outward through the cover in the manner of a rivet. The bottom of the lower part serves as the further countercontact for the switching mechanism.
Since the spring disk itself is produced from electrically conductive material, below the response temperature of the switching mechanism it ensures a low-resistance electrically conductive connection between the countercontact on the cover part and the countercontact on the lower part, contact being made to the lower part from outside. If the temperature of the switching mechanism then rises, the bimetallic snap disk suddenly snaps over and pushes the movable contact element, against the force of the spring disk, away from the countercontact of the cover so that the electrical connection is interrupted.
Switches of this kind are commonly used for temperature monitoring of electrical devices. As long as the temperature of the electrical device does not exceed a predetermined response temperature, the switch, which for this purpose is connected in series with the load to be protected, remains closed. If the temperature of the load then rises impermissibly, the bimetallic snap disk snaps over and thus interrupts the flow of current to the load.
A disadvantage of the known switch is that it is relatively complex to manufacture. This is due principally to the fact that after manufacture of the cover part, the countercontact must then be attached to the cover part, and provision must simultaneously be made for an electrically conductive connection through the wall of the cover part to the outside. This is accomplished in the manner of a rivet which transitions, outside the cover, into a head onto which conductors, crimp terminals, etc. can be soldered. This assembly of the countercontact to the cover part is generally accomplished manually, and is thus very cost-intensive.
A further switch, in whose housing a temperature-dependent switching mechanism as described above is also arranged, is known from DE 21 21 802 A1. In this switch, the cover part and lower part are both cup-shaped and are produced from electrically conductive material. Crimp terminals are integrally shaped onto both the upper part and the lower part, the crimp terminal of the lower part extending outward through a corresponding cutout in the wall of the upper part. An insulating film is arranged between the upper part and the lower part in order to insulate the two housing parts electrically from one another.
The temperature-dependent switching mechanism makes contact on the one hand with the lower part via the spring disk, and on the other hand with the cover part via the movable contact element, so that an electrically conductive connection exists between the two crimp terminals as long as the temperature of the switching mechanism is below the response temperature. If the temperature of the switching mechanism rises, this electrical connection is interrupted in the manner described above.
With this switch as well, final assembly is very complex due to the insulation film that must be set in place, and can thus only be performed manually. This manual final assembly is not only wage-intensive, but also leads to assembly errors and thus to a higher rejection rate.
A further disadvantage of the two switches described so far is that for certain applications they must additionally be insulated externally, since current flow occurs through the electrically conductive lower part.
U.S. Pat. No. 4,490,704 discloses a further temperature-dependent switch which has a lower housing part made of insulating material and a cover made of metal which rests on a shoulder of the lower part and is retained by a rim of the lower part. The temperature-dependent switching mechanism comprises a bimetallic spring, clamped at one end, which at its free end holds a movable contact which, below the response temperature of the switching mechanism, is in contact with a fixed countercontact that is arranged internally on the cover.
At its other end the bimetallic spring is immovably clamped and connected to a resistor which runs along the bottom of the lower part. Provided in the bottom is a through hole into which a button-like connector element is inserted from below. This connector element is soldered, at its head projecting into the interior of the switch, to the resistor. The button-like head transitions into a clip which runs through laterally under the wall of the lower part and transitions into a connector lug next to the lower part.
This document therefore describes a completely different temperature-dependent switching mechanism from the two publications cited above, in which, because of the bimetallic spring clamped at one end, lesser demands are made in terms of insulation of the switching mechanism in the various switching states.
Making contact to the clamped end of the bimetallic spring is very laborious due to the button-like connector element: not only are parts of very complex shape necessary, but because the button-like head is soldered to the resistor in the interior of the lower part, assembly is very laborious. A further disadvantage of this switch is that is not insulated either at the top or at the bottom, so that particular precautionary measures are necessary when it is attached to a device being protected.
A further disadvantage with the two switches described at the outset arises from the fact, desirable in itself, that the bimetallic snap disk is not mechanically loaded and is placed, so to speak, unconstrainedly into the housing, so that mechanical loads cannot lead to a shifting of the switching temperature, as is the case with the switch, also discussed, having the bimetallic spring clamped at one end. However, when switches with bimetallic snap disks laid in loosely in this fashion are used in the vicinity of alternating magnetic fields, the bimetallic snap disk can be caused to vibrate since it can be magnetized by a magnetic field because of its composition. In other words, the bimetallic snap disk is magnetized by the external magnetic field and follows its oscillations.
Such vibrations of the bimetallic snap disk are, however, undesirable, since they mechanically stress it, which can lead to a shortening of service life and uncontrolled shifting of the switching temperature. In order to suppress such influences, temperature-dependent switches of the kind mentioned at the outset are therefore often equipped with a magnetic shielding plate; it is furthermore known to set in place a further stabilizing disk which retains the bimetallic snap disk in vibration-free fashion below its response temperature. This additional retaining of the bimetallic snap disk, however, on the one hand is structurally complex and on the other hand has the undesirable side effect that the bimetallic snap disk is in fact being loaded, which was precisely what loose placement is intended to prevent.
In this context it is known from DE 196 36 320 A1, in the case of a temperature-dependent switch having a spring tongue clamped at one end and a bimetallic strip set unconstrainedly in place, to produce a retaining or guidance element of the spring tongue and/or bimetallic strip from magnetic material. Here, in a manner known per se, the spring tongue carries a movable contact which is in contact with a fixed contact, such that the spring tongue (at its clamped-in ends) and the fixed contact are each connected to an external terminal. In the event of an impermissible rise in temperature, the bimetallic strip--which is set loosely in place but is guided at its narrow ends--lifts the movable contact away from the fixed contact.
The magnetized retaining or guidance element is located next to one of the two narrow sides of the bimetallic strip; an alternating magnetic attraction between the bimetallic strip and the retaining or guidance element is intended to prevent vibrations of the bimetallic strip.
With this switch as well, on the one hand its complex design is disadvantageous: here again, the mechanical clamping at the retained end of the spring tongue influences the switching temperature unpredictably. A further disadvantage with this switch is that the attractive force between the retaining and guidance element and the bimetallic strip is often not sufficient, since these two elements are in a geometrically very unfavorable position with respect to one another. The desired suppression of vibrations is therefore often not achieved with this switch.